Gasoline is comprised of a complex mixture of volatile hydrocarbons which is suitable for use as a fuel in a spark-ignition internal combustion engine, and it typically boils over a temperature range of about 80.degree. to about 437.degree. F. Although gasoline can consist of a single blendstock, such as the product from a refinery alkylation unit, it is usually comprised of a blend of several blendstocks. The blending of gasoline is a complex process, which typically involves the combination of from as few as three or four to as many as twelve or more different blendstocks to meet regulatory requirements and such other specifications as the manufacturer may select. Optimization of this blending process must take into account a plurality of characteristics of both the blendstocks and the resulting gasoline. Among others, such characteristics can include cost and various measurements of volatility, octane, and chemical composition.
It is conventional practice in the industry to blend gasoline using blendstock ratios which are determined by mathematical algorithms which are known as blending equations. Such blending equations are well known in the refining industry, and are either developed or tailored by each refiner for use in connection with available blendstocks. Blending equations typically relate the properties of a gasoline blend to the quantity of each blendstock in the blend and also to either the measured or anticipated properties of each blendstock in the blend.
Although hydrocarbons usually represent a major component of gasoline, it has been found that certain oxygen containing organic compounds can be advantageously included as gasoline components. These oxygen containing organic compounds are referred to as oxygenates, and they are useful as gasoline components because they are usually of high octane and may be a more economical source of gasoline octane than a high octane hydrocarbon blending component such as alkylate or reformate. Current government regulation in the U.S. limits the oxygen content of gasoline to 4.0 wt. % and also requires that reformulated gasolines contain at least 1.5 wt. % of oxygen. Oxygenates which have received substantial attention as gasoline blending agents include ethanol, t-butyl alcohol, methyl t-butyl ether, ethyl t-butyl ether, and methyl t-amyl ether. However, ethanol has become one of the most widely used oxygenates.
Ethanol is not usually blended into a finished gasoline within a refinery because the ethanol is water soluble. As a consequence of this solubility, an ethanol-containing gasoline can undergo undesirable change if it comes in contact with water during transport through a distribution system, which may include pipelines, stationary storage tanks, rail cars, tanker trucks, barges, ships and the like. For example, an ethanol-containing gasoline can absorb or dissolve water which will then be present as an undesirable contaminant in the gasoline. Alternatively, water can extract ethanol from the gasoline, thereby changing the chemical composition of the gasoline and negatively affecting the specifications of the gasoline.
In order to avoid, as much as possible, any contact with water, ethanol-containing gasoline is usually manufactured by a multi-step process wherein the ethanol is incorporated into the product at a point which is near the end of the distribution system. More specifically, gasoline which contains a water soluble alcohol, such as ethanol, is generally manufactured by producing an unfinished and substantially hydrocarbon precursor blend at a refinery, transporting the unfinished blend to a product terminal in the geographic area where the finished gasoline is to be distributed, and mixing the unfinished blend with the desired amount of alcohol at the product terminal. A substantially hydrocarbon precursor blend which can be converted to a finished gasoline by mixing with one or more alcohols is referred to herein either as a "subgrade" or as a "subgrade blend." The combination of the subgrade with the alcohol yields a finished gasoline which meets all specifications for sale. The subgrade is commonly called a RBOB (Reformulated Blendstock for Oxygenate Blending) when the subgrade is destined for a reformulated gasoline market in the U.S.
When a subgrade is manufactured at a refinery, the subgrade's properties are measured and controlled to intermediate specifications that differ from the finished gasoline. Intermediate specifications are use to compensate for the effects of alcohol which will be added to the subgrade after it leaves the refinery. However, the effects of alcohols such as ethanol and methanol are variable and depend on the chemical composition of the subgrade. For example, the addition of ethanol has a substantial effect on gasoline volatility, and the magnitude of this effect is dependent on the chemical composition of the subgrade blend. The addition of ethanol to gasoline affects the distillation curve of the resulting product by reducing the evaporation temperatures of the front end, which affects primarily the first 50% evaporated. Ethanol generally depresses the boiling point of aromatic hydrocarbons slightly less than that of aliphatic hydrocarbons. In addition, blending ethanol into gasoline results in a nonideal solution that does not follow linear blending relationships. Rather than lowering the vapor pressure of the resulting blend, ethanol causes an increase in the vapor pressure.
The variable and somewhat unpredictable effects which result when an alcohol, such as ethanol, is mixed with a subgrade blend to form a finished gasoline are taken into account by setting more stringent specifications for the finished gasoline than are ordinarily required. These more stringent specifications include a margin for error to accommodate the variable effect of the alcohol. Because of the margin for error, the desired specifications for the finished gasoline are usually exceeded. Unfortunately, this can add cost to the manufacturing process since expensive blendstocks may be required to achieve the margin for error.
Ethanol-free gasoline is typically produced within a refinery as a finished product which fully meets all necessary specifications for sale. This finished gasoline can be manufactured to very precisely fit the specifications because analytical data for the product can be used to control the blending process. As a consequence, manufacturing costs are kept to a minimum because expensive blendstocks are never wasted through exceeding specifications. Unfortunately, this type of precise manufacturing control is not presently possible with respect to an ethanol-containing gasoline which is prepared by mixing a subgrade blend with ethanol.